Draft inducer performance control

ABSTRACT

A process for controlling the performance of a draft inducer. A desired carbon dioxide and a carbon monoxide level are selected and associated with a desired temperature of a mixed flow of exhaust fumes and ambient air. The temperature of the mixed flow is measured and the flow of ambient air is controlled to maintain the temperature of the mixed flow at substantially the desired temperature. The speed of the draft inducer blower will increase or decrease so as to maintain the desired temperature.

PRIORITY

The present application claims priority from U.S. provisional application Ser. No. 60/499,844, filed Sep. 3, 2003 and hereby incorporated by reference in its entirety.

RELATED APPLICATIONS

The present application is related to co-pending U.S. patent application Ser. No. ______ filed on even date therewith, entitled “Apparatus and Method for Maintaining an Operating Condition for a Blower” and naming Fred A. Brown as the inventor, the disclosure of which is incorporated herein, in its entirety, by reference.

FIELD OF THE INVENTION

The invention generally relates to air movement and, more particularly, the invention relates to draft inducers.

BACKGROUND OF THE INVENTION

Fuel burning furnaces commonly have an attached draft inducer that mixes ambient air with exhaust fumes produced by burning fuel. After the exhaust fumes and ambient air are mixed, they are vented from the furnace through an exhaust pipe. Among other benefits, draft inducers can improve heater efficiency by controlling the flow of exhaust fumes from their furnaces.

More particularly, when exhausting fumes, inducers necessarily draw heat away from their corresponding furnace. Idealized inducers draw exactly the minimum amount of fumes (and thus, the minimum amount of heat) to ensure that carbon monoxide is sufficiently exhausted. For example, an inducer that draws excess air from a hot water heater necessarily increases the amount of time required to heat the water. Accordingly, such an inducer wastes energy. Conversely, carbon monoxide can build up if the inducer does not draw enough air from the heater. This clearly is a very dangerous condition and thus, must be avoided. Accordingly, some prior art inducers inefficiently draw too much air to avoid this dangerous build-up condition.

By mixing exhaust fumes with ambient air to create an exhaust/air mixture, a draft inducer lowers the concentration of carbon dioxide and carbon monoxide. An elevated concentration of carbon dioxide can result in drowsiness, and prolonged exposure to a consequently oxygen deficient environment can lead to suffocation. Exposure to elevated concentrations of carbon monoxide is more dangerous. By inactivating hemoglobin within red blood cells, carbon monoxide may cause death even in the presence of an otherwise adequate oxygen concentration.

One approach to maintaining carbon dioxide and carbon monoxide levels in the exhaust/air mixture below thresholds known to cause problems is to control the speed of the draft inducer on the basis of those concentrations. The proper draft creates the proper combustion efficiency. Optimum combustion is consistent with low carbon dioxide and carbon monoxide concentrations. The higher the speed, the more ambient air added, and the lower the concentration of the unwanted constituents. However, measurement of gas concentrations may be relatively expensive and complex as gas sensors may be costly and exposure of sensors to an exhaust/air mixture may require protecting sensors or withdrawing a sample of the exhaust/air mixture.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, the composition of the output of a draft inducer is controlled by associating a desired temperature with desired concentrations of one or more constituents of the draft inducer output and controlling an air flow so as to substantially maintain the desired temperature. Optionally, the constituents may be carbon dioxide, carbon monoxide, and water. The controlling of an air flow may be accomplished with a look up table. In certain cases, the temperature at the output of the draft inducer may be measured, as with a thermistor. The desired temperature to associate with the desired concentrations may also be determined. On occasion, an audible signal may be generated, as by a speaker or by a piezo alarm.

In some embodiments, a desired temperature may be associated with desired concentrations of constituents and with an ambient temperature. Association may be accomplished by accessing a look up table. The temperature of ambient air entering the draft inducer, as well as the temperature of the draft inducer output, may be measured, as with thermistors.

In accordance with another aspect of the invention, a system for controlling the composition of the output of a draft inducer includes means for associating a desired temperature of the draft inducer output with one or more desired concentrations of constituents and means for controlling an air flow so as to substantially maintain the desired temperature. Certain embodiments may include a microprocessor where the microprocessor may include a memory and the memory may include a look up table that may include the desired temperature and the desired concentrations of the constituents. The constituents may include carbon dioxide, carbon monoxide, and water. The temperature of the draft inducer output may be measured with a thermistor.

Certain embodiments may include means for measuring the temperature of ambient air entering the draft inducer, as with a thermistor. There may be a microprocessor where the microprocessor may include a memory and the memory may include a look up table that may include the desired temperature, the desired concentrations of the constituents, and the ambient temperature. The constituents may include carbon dioxide, carbon monoxide, and water. Some embodiments may include means for generating an audible signal, as with a speaker or with a piezo alarm.

In accordance with a further aspect of the invention, a draft inducer capable of coupling with an exhaust pipe includes an input for detecting a temperature of a draft inducer output at a selected location in the exhaust port of the draft inducer, an air moving device, capable of rotating at a plurality of different rotational speeds, for generating air flow in the exhaust pipe, and a control module for controlling the rotational speed of the air moving device as a function of the difference between the temperature of the draft inducer output in the exhaust pipe and a desired temperature reflective of desired concentrations of constituents, including carbon dioxide, carbon monoxide, and water in the exhaust pipe.

In certain embodiments, the control module may control the rotational speed of the air moving device to maintain the temperature in the exhaust pipe at substantially the desired temperature. The draft inducer output may be coupled to the exhaust pipe at the draft inducer exhaust port. In some embodiments, the draft inducer may include an audible alarm, such as a piezo alarm or a speaker.

In accordance with an additional aspect of the invention, a computer program product for use on a computer system for controlling the composition of the output of a draft inducer includes a computer usable medium having computer readable program code. The computer readable program code contains program code for associating a desired temperature with desired concentrations of one or more constituents of the draft inducer output and program code for controlling an air flow so as to substantially maintain the desired temperature. In some embodiments, the desired temperature may be associated with desired carbon dioxide, desired carbon monoxide, and water concentrations. There may be program code for measuring a temperature of draft inducer output and for accessing a look up table.

In another embodiment, there may be program code for associating a desired temperature with a desired carbon dioxide concentration, with a desired carbon monoxide concentration, with a desired water concentration and with the ambient temperature. There may also be program code for measuring draft inducer output temperature and for determining the desired temperature to associate with the desired concentration.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and advantages of the invention will be appreciated more fully from the following further description thereof with reference to the accompanying drawings wherein:

FIG. 1 schematically shows a draft inducer system that may be configured in accordance with illustrative embodiments of the invention.

FIGS. 2A and 2B schematically show a draft inducer configured in accordance with illustrative embodiments of the invention.

FIG. 3 schematically shows the electronic configuration and connections between the draft inducer and a corresponding hot water heater.

FIG. 4 shows an exemplary process executed by the electronic circuit shown in FIG. 3.

FIG. 5 schematically shows a processor system with various connections to other circuit elements within the circuit shown in FIG. 3.

FIGS. 6A and 6B shows experimental maintenance of gas concentration for various lengths of 2 inch and 3 inch diameter exhaust pipes.

FIG. 7 shows an example of correction of desired exhaust/air mixture temperature for ambient temperatures.

FIG. 8 shows a look-up table that may be used by the processor system shown in FIG. 5.

FIG. 9 shows a process used by the processor system to control carbon dioxide and carbon monoxide concentrations.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

An indirect approach may permit control of constituent concentrations within the exhaust from a combustor of carbon-based fuels more simply and less expensively. Specifically, for a given ambient air temperature, one may associate, in illustrative embodiments, desired concentrations of carbon dioxide and carbon monoxide with a desired temperature of the exhaust/air mixture measured at an exhaust port immediately after mixing ambient air and exhaust in a mixing chamber. As a result, control of draft inducer speed to produce the desired mixture temperature may also control the composition of the exhaust/air mixture, thereby limiting carbon dioxide and carbon monoxide concentrations to desired values. In addition, measurement of mixture temperature may employ a relatively inexpensive and easy to install sensor, such as a thermistor or a thermocouple.

FIG. 1 schematically shows a draft inducer system (the “system 10”) that may be configured in accordance with illustrative embodiments of the invention. The system 10 has a fuel burning device 12 with a draft inducer 14 mounted to its top side. Among other things, the draft inducer 14 forcibly vents exhaust fumes produced by the burning fuel.

In illustrative embodiments, the fuel burning device 12 is a conventional natural gas hot water heater (also identified by reference number 12). Accordingly, various embodiments are discussed as being used with a natural gas hot water heater 12. Rather than having a hot water heater 12, however, the system 10 may have other types of fuel burning devices, such as boilers. Discussion of a hot water heater 12 thus is exemplary and, consequently, not intended to limit all embodiments of the invention. Moreover, various embodiments may be implemented with other types of burnable fuels, such as oil or wood. Accordingly, discussion of burning natural gas is not intended to limit all embodiments of the invention.

The draft inducer 14 illustratively is mounted over an exhaust opening in the hot water heater 12 to receive exhaust fumes produced by the burning fuel. In illustrative embodiments, that mounting is between a cold water intake pipe 16 and a hot water outlet pipe 18 at the top of the heater 12. When the system 10 is on, the draft inducer 14 mixes the exhaust fumes with ambient air within its interior, and forces the resultant draft inducer output or exhaust/air mixture 90 out of the premises via an exhaust pipe 20. Because the resultant mixture typically has a much lower temperature than that of the exhaust fumes, the exhaust pipe 20 can be produced from a lower cost material, such as conventional PVC piping.

An AC plug 22 that can be inserted into a conventional AC outlet powers the overall system 10. Accordingly, installation requires no specialized electrical expertise. Draft inducer 14 also contains a connection plug 42 to provide electrical power to hot water heater 12.

FIGS. 2A and 2B schematically shows a draft inducer 14 configured in accordance with illustrative embodiments of the invention. The draft inducer 14 has a metal or plastic housing 24 (including sections 24A and 24B held together by compression clips 52) containing a blower 25, a blower controller 27, and a step-down transformer 36 and forming an exhaust port 26 for directing the noted exhaust/air mixture 90 of exhaust fumes and ambient air to the exhaust pipe 20. The housing 24 also forms an interior mixing chamber 28 for mixing ambient air with exhaust fumes. One or more ambient air inlets 30 formed in the side of the housing 24 permit the blower 25 to draw ambient air into the mixing chamber 28, thus enabling the exhaust to mix with the air. A mixture thermistor 78 for measuring the temperature of the output 90 of the draft inducer 14, i.e., the exhaust/air mixture, and an over-temperature switch 72 are mounted in the exhaust port 26 of the inducer 14 in FIG. 2B. An ambient air thermistor 80 for measuring the temperature of the ambient air may be mounted within ambient air inlet 30.

Plug 22, which brings AC wall voltage to draft inducer 14 through electrical cord 38 has a pair of prongs 34 for mating with a wall plug (e.g., a home AC outlet, such as those in North America and Europe, or a power strip). The plug 22 may be produced from any conventional material used for these purposes, such as a hard or soft plastic.

FIG. 3 schematically shows the general electronic configuration and connections between the draft inducer 14 and the hot water heater 12. The various parts of the system 10 are divided by vertical dashed lines. In particular, relevant components of the draft inducer 14 (i.e., noted as being within the housing 24 and/or exhaust pipe 20) are shown and identified by reference number 14, and relevant components of the heater 12 are shown and identified by reference number 12.

The step-down transformer 36 receives the wall voltage (e.g., 115 volts AC) from plug 22 and produces a stepped-down AC voltage (e.g., 24 volts AC). The stepped-down voltage then is fed to a bridge rectifier 64 (within the draft inducer housing 24), which converts the stepped-down AC voltage to a DC voltage (e.g., 24 volts DC).

The draft inducer 14 also includes a first capacitor C1 that reduces ripple from the bridge rectifier 64 and stores the DC voltage received from the bridge rectifier 64. A voltage regulator 24 maintains a 24 volt voltage at the input to the blower controller 27. A second capacitor C2 at the output of voltage regulator 24 maintains stable regulator operation. The second capacitor C2 does not provide any appreciable filtering. In alternative embodiments, the second capacitor C2 is omitted.

The blower controller 27 may include one or more program leads (shown schematically at reference number 66) that permit it to perform in pre-specified ways. For example, as noted above, the blower controller 27 may be programmed to cause the blower 25 to rotate at a speed (i.e., in RPMs) that is based upon the temperature of the exhaust/air mixture in the exhaust port 26. To that end, the blower 25 and the blower controller 27 may include hardware and/or software that perform the noted functions. For example, the blower controller 27 may include a processor system 68 that executes a specified function based upon computer instructions. Different hardware within the blower 25 and the blower controller 27 also may be configured to perform the noted functions. Additional details of the processor system 68 and some of its functions are discussed below with reference to FIGS. 5-7.

The blower controller 27 includes internal circuitry (e.g., the processor or control module 68) that produces a signal having a zero output state (e.g., logic level of zero) when the blower 25 is running (i.e., rotating at or above a pre-selected RPM value), and an open output state (e.g., logic level one) when the blower 25 is not running. This signal is transmitted directly to a switch 69 (via a resistor R7) that, consequently, is open when the blower 25 is off, and closed when the blower 25 is on. In illustrative embodiments, the switch 69 is a NPN bipolar junction transistor (“BJT”) that is biased on when its base receives the noted zero output state.

The draft inducer 14 also includes an over temperature switch (“OTS 72”) to ensure that the temperature of the exhaust/air mixture in the exhaust port 26 does not exceed a prescribed temperature. The inducer 14 further has a mixture thermistor sensor 78 to measure the temperature of the exhaust/air mixture, and may have an ambient air thermistor sensor 80 to measure the temperature of the ambient air. To that end, if the OTS 72 detects that the exhaust/air mixture exceeds such temperature, it opens, thus breaking the circuit to the heater 12. As a consequence, the heater 12 cannot continue to operate. The exhaust/air mixture temperature and ambient air temperature, if available, are used to control the blower speed to maintain a desired exhaust/air mixture temperature.

The heater 12 has a solenoid 74 that turns on the gas supply when on, and turns off the gas supply when off. Accordingly, when either the OTS 72 or switch 69 breaks the circuit, the solenoid 74 cannot function, consequently cutting off the supply of gas to the heater 12. Consequently, the heater 12 cannot burn gas, which produces exhaust fumes.

The heater 12 also includes a low temperature switch (“LTS 76”), which selectively turns the heater 12 on and off based upon water temperature within the tank. Specifically, the LTS 76 turns the heater 12 on (i.e., it closes) when it detects that the water temperature is below a pre-selected low temperature. In a corresponding manner, the LTS 76 turns the heater 12 off (i.e., it opens) when it detects that the water temperature is at or above a pre-selected high temperature.

FIG. 4 shows an exemplary process executed by the electronic circuit shown in FIG. 5. The process begins at step 500, in which the low temperature switch 76 is closed to begin heating water in the tank. This connection provides the ground return for the blower 25, which causes it to begin rotating.

It then is determined at step 502 if the blower 25 is running at a predetermined minimum speed that can provide a sufficient pressure to perform the draft inducing function. This predetermined speed may be based upon a number of variables, such as the size of the hot water heater 12, the size of the blower 25 and inducer 14, and other known parameters associated with the system 10. In illustrative embodiments, the processor 68 makes this determination. The hot water heater 12 thus receives no power until the blower 25 reaches this minimum speed.

After the blower 25 reaches the minimum speed, the process continues to step 504, in which the solenoid 74 is connected to the power source. To that end, the processor 68 forwards a signal to the switch 69 (e.g., a logical zero). Upon receipt, the switch 69 closes, which connects the power source (i.e., the transformer 36 and rectifier 64) with the solenoid 74. Consequently, the solenoid 74 turns on the gas supply, which enables the hot water heater 12 to begin heating the water in its internal tank. After a certain delay from the time at which the blower reaches the minimum speed, the system increments the blower speed to create a temperature at the exhaust port 26 reflective of desired carbon dioxide and carbon monoxide concentrations.

The hot water heater 12 continues to heat the water until it reaches a predetermined maximum temperature (step 506). When that temperature is reached, the hot water heater 12 opens its low temperature switch 76 (step 508), which effectively breaks the return path for the rectifier 64.

The water in the hot water heater 12 then cools until it reaches a predetermined minimum temperature (step 510). After it reaches the minimum temperature, the process repeats by looping back to step 500, in which the low temperature switch is closed.

It should be noted that this process can be interrupted at any time for several reasons. Primarily, if the blower 25 malfunctions by not rotating rapidly enough, the switch 69 opens, which stops the hot water heater 12 from burning its fuel. In other words, if the blower 25 does not reach its predetermined speed, then the entire system 10 effectively shuts down. In such case, the blower 25 may be replaced. If the various parameters are properly selected, then this feature should help prevent the hot water heater 12 from generating more exhaust fumes than the inducer 14 can force from the system 10. The process also can be interrupted if the exhaust/air mixture is too hot, which causes the over temperature switch 42 to open.

In illustrative embodiments of the invention, the processor system 68 controls the speed of the blower 25 to ensure that the carbon dioxide and carbon monoxide concentrations in the exhaust port 26 are maintained at substantially optimum gas concentration values.

To those ends, FIG. 5 shows the processor system 68 with various connections to other circuit elements within the circuit shown in FIG. 3. In particular, the processor system 68 includes a commutation processor U2 for controlling normal blower commutation, and a control processor U1 for controlling the speed of the blower as a function of the temperature of the exhaust/air mixture within the exhaust port 26. In illustrative embodiments, the control processor U1 is a PIC processor (flash microprocessor), such as model number 16F818 from Microchip, Inc., while the commutation processor U2 is an INFINEON POWERCHIP type of H-Bridge motor drive (e.g., model number 6LE6209), from Infineon.

FIG. 5 shows a number of other components, such as the bridge rectifier 64 (discussed above), various capacitors and resistors, and a speaker 81. The speaker 81 may sound an audible signal if an error condition is detected to inform an operator that something is not correct. (A piezo alarm may also be used to generate the audible signal.) For example, a signal corresponding to two beeps may indicate that the blower motor is running too fast, that there may be a blockage, that the water heater should not be used, and that service should be contacted. Three beeps may indicate that the blower motor is running two slowly and that service should bring a replacement blower.

The control processor U1 includes memory for storing, among other things, instructions relating to its operation, a look up table relating ambient air temperature with desired exhaust/air mixture temperatures, and a look up table with information for controlling the blower speed (in RPMs) as a function of duty cycle.

It has been experimentally determined (FIG. 6) that combinations of concentrations of carbon dioxide and carbon monoxide for a specific type of hot water heater (Bradford White 40 gallon gas hot water heater Model MITW 40L6BN12) can be associated with exhaust/air mixture temperatures at the exhaust port 26 of the draft inducer 14. An illustrative correlation between the desired exhaust/air mixture temperature measured at the outlet of the exhaust port 26 and desired carbon dioxide and carbon monoxide concentrations there is shown in FIG. 7A for hot water heater (Bradford White 40 gallon gas hot water heater Model MITW 40L6BN12). It has been found that a desired carbon dioxide concentration of 9 ppm and a desired carbon monoxide concentration of 30 ppm may be maintained in a 2 inch diameter exhaust pipe, independent of the length of the pipe, if the speed (i.e., as measured in revolutions per minute or RPM) of the blower is adjusted to maintain the temperature of the exhaust/air mixture at 170° F., as measured at the exhaust port 26. It has also been found that the adjustment of blower speed required to maintain constant exhaust/air temperature consistent with desired carbon dioxide and carbon monoxide concentrations also maintains constant volume flow rate (i.e., as measured in cubic feet per minute or CFM) of the exhaust/air mixture. FIG. 7B illustrates maintenance of a carbon dioxide concentration of 9 ppm and a carbon monoxide concentration of 30 ppm with an exhaust/air temperature of 140° F. for a 3 inch diameter exhaust pipe. It has been observed that control of the carbon monoxide concentration may become problematic for carbon monoxide concentrations in excess of 30 ppm as the carbon monoxide concentration becomes very sensitive to changes in blower speed.

Correlation between desired exhaust/air mixture temperatures and the desired carbon dioxide and carbon monoxide concentrations may be somewhat affected by the ambient temperature. Additional control for ambient temperature may be achieved by adjustment of the desired temperature associated with desired concentrations of carbon dioxide and carbon monoxide. For example, for a desired carbon dioxide concentration of 9 ppm and a desired carbon monoxide concentration of 30 ppm, FIG. 7 illustrates the relationship between desired exhaust/air mixture temperatures and ambient air temperatures of 60° F., 70° F., 80° F., and 90° F.

In general, the lower the desired exhaust/air mixture temperature, the higher are the desired concentrations of carbon dioxide and carbon monoxide. Consequently, control of blower RPM to control exhaust/air mixture temperature also controls carbon dioxide and carbon monoxide concentrations by altering the proportions of exhaust and ambient air in the exhaust/air mixture. To that end, for the specific type of hot water heater (Bradford White 40 gallon gas hot water heater Model MITW 40L6BN12) a number of RPM values have been associated with different duty cycles (which control blower speed) for exhaust pipe lengths of 5, 10, and 20 feet in FIG. 8. This data may be stored as a look-up table in processor system 68.

In operation, once the blower 25 has been determined to be running at a predetermined speed, the blower speed may be increased or decreased and the exhaust/air mixture temperature may be monitored by thermistor 78. As long as the temperature indicated by the thermistor 78 is below the desired temperature, in this case, 170° F., the duty cycle may be increased to increase the blower speed. When the temperature derived from thermistor 78 matches the desired temperature, the blower speed is kept constant. Beginning at a moderate speed and increasing rather than beginning at a high blower speed and decreasing minimizes the disturbing effect that the sound from a suddenly energized blower may have. Another reason to start the blower 25 at a moderate speed is avoidance of sooting.

As an example, to obtain a carbon dioxide concentration of 9 ppm (parts per million) or less and a carbon monoxide concentration of 30 ppm or less in exhaust pipe 20 of a Bradford White 40 gallon gas hot water heater (Model MITW 40L6BN12), the temperature of the exhaust/air mixture may be controlled to be 170° F., independent of the length of exhaust pipe 20, within the capability of the blower 25. Different combinations of desired carbon dioxide and carbon monoxide concentrations may be associated with different desired exhaust/air temperatures and may be contained in a look up table.

During operation, the processor U1 sets the duty cycle at one of the duty cycles in the table to establish a speed for blower 25. Hall effect sensor 90 gives feedback to processor U1 to define blower speed (RPM). If the exhaust/air mixture temperature does not match the desired exhaust/air mixture temperature, then the processor U1 changes the duty cycle to another value in the table. For example, the processor U1 increases the RPM (i.e., increases the duty cycle) if the exhaust/air mixture temperature is below the desired exhaust/air mixture temperature, or decreases the RPM (i.e., decreases the duty cycle) if the exhaust/air mixture temperature is above the desired exhaust/air mixture temperature. After increasing or decreasing the RPM, the processor U1 again checks the exhaust/air mixture temperature. This process iterates until there is a match between the measured exhaust/air temperature and the desired exhaust/air temperature. Details of the use of this table are discussed below with reference to FIG. 9.

The process of FIG. 9 begins at step 800, where the initial duty cycle is set. The processor U1 sets the initial duty cycle to a middle entry in the look up table appropriate for the length of exhaust pipe 20 to minimize sooting. For example, the duty cycle may be set to DC8 for a 10 foot long exhaust pipe. In alternative embodiments where, for example, sooting is not a concern, the initial duty cycle may be the first entry in the look up table (DC6 in the case of a 10 foot long exhaust pipe).

Within seven seconds, ignition is initiated in step 803. A time limit is imposed to avoid having excessive fuel vapor in the exhaust pipe 20. If a time limit is not imposed, ignition may result in an explosion.

It then is determined at step 804 if the actual exhaust/air mixture temperature indicated by mixture thermistor 78 in exhaust port 26 associated with the currently set duty cycle is higher or lower than the desired exhaust/air mixture temperature associated with the desired carbon dioxide and carbon monoxide concentrations. (For additional control, the desired exhaust/air mixture temperature may also be based upon the current ambient temperature.) In the example above, it is determined if the actual exhaust/air mixture temperature exceeds 170° F. If the actual exhaust/air temperature is lower, then the process continues to step 806, in which it is determined if the duty cycle is at a maximum value in the table. If it is at the maximum value in the table, (which, in the current example, is not the case), then the process continues to step 808, in which the system 10 is shut down. In such case, an error condition may have occurred, such as the blockage of the exhaust pipe 20, such as a nest built on the outlet. To that end, the processor U1 shuts down the blower 25, which causes the solenoid 74 to shut down.

Conversely, if at step 806 it is determined that the duty cycle is not at the maximum value, then the process continues to step 810, in which the duty cycle is incremented by one look up table entry. Accordingly, at step 810, the duty cycle is increased from DC8 to DC9. The process then loops back to step 804.

Returning to step 804, rather than determining that the actual exhaust/air mixture temperature is lower, if the actual exhaust/air mixture temperature is higher than the desired exhaust/air mixture temperature 170° F., then the process continues to step 812. At that step, it is determined if the duty cycle is at a minimum value in the table. If at the minimum value, then the system 10 shuts down (as discussed above-step 808). This type of shut down could suggest some disconnection of the exhaust pipe 20. If the current entry is for DC8, however, then the minimum duty cycle has not been reached.

If the duty cycle is not at the minimum, as in the example, then the process continues to step 814, in which the duty cycle is decremented. Accordingly, if the current duty cycle is DC8, then the duty cycle is decremented to the next entry in the table, which is DC7. The process then loops back to step 804.

Of course, if the processor U1 detects a match at step 804 (i.e., the actual exhaust/air mixture temperature equals the desired exhaust/air mixture temperature), then the processor U 1 remains at step 804 until it detects a difference.

Those skilled in the art should understand that if a fan is used instead of a blower, then step 810 would decrement the duty cycle, while step 814 would increment the duty cycle. This occurs because a blower generally rotates faster as impedance increases. Indeed, such a reaction is opposite to that of a fan, which rotates slower as impedance increases.

Various embodiments of the invention may be implemented at least in part in any conventional computer programming language. For example, some embodiments may be implemented in a procedural programming language (e.g., “C”), or in an object oriented programming language (e.g., “C++”). Other embodiments of the invention may be implemented as preprogrammed hardware elements (e.g., application specific integrated circuits, FPGAs, and digital signal processors), or other related components.

In an alternative embodiment, the disclosed apparatus and method may be implemented as a computer program product for use with a computer system. Such implementation may include a series of computer instructions fixed either on a tangible medium, such as a computer readable medium (e.g., a diskette, CD-ROM, ROM, or fixed disk) or transmittable to a computer system, via a modem or other interface device, such as a communications adapter connected to a network over a medium. The medium may be either a tangible medium (e.g., optical or analog communications lines) or a medium implemented with wireless techniques (e.g., WIFI, microwave, infrared or other transmission techniques). The series of computer instructions can embody all or part of the functionality previously described herein with respect to the system.

Those skilled in the art should appreciate that such computer instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Furthermore, such instructions may be stored in any memory device, such as semiconductor, magnetic, optical or other memory devices, and may be transmitted using any communications technology, such as optical, infrared, microwave, or other transmission technologies.

Such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation (e.g., shrink wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server or electronic bulletin board over the network (e.g., the Internet or World Wide Web). Of course, some embodiments of the invention may be implemented as a combination of both software (e.g., a computer program product) and hardware. Still other embodiments of the invention are implemented as entirely hardware, or entirely software.

Although various exemplary embodiments of the invention are disclosed above, it should be apparent that those skilled in the art can make various changes and modifications that will achieve some of the advantages of the invention without departing from the true scope of the invention. 

1. A method for controlling the composition of the output of a draft inducer, the method comprising: associating a desired temperature with desired concentrations of one or more constituents of the draft inducer output; and controlling an air flow so as to substantially maintain the desired temperature.
 2. The method of claim 1, wherein associating a desired temperature with desired constituent concentrations includes associating a desired temperature with a desired carbon dioxide concentration, with a desired carbon monoxide concentration, and with a desired water concentration.
 3. The method of claim 1, wherein controlling an air flow includes measuring a temperature of the output of the draft inducer.
 4. The method of claim 3, wherein measuring the temperature of the output of the draft inducer includes measuring with a thermistor.
 5. The method of claim 1, wherein controlling an air flow includes accessing a look up table.
 6. The method of claim 1, further including measuring the temperature of ambient air entering the draft inducer.
 7. The method of claim 6, wherein measuring the temperature of ambient air includes measuring with a thermistor.
 8. The method of claim 6, wherein associating a desired temperature with one or more desired constituent concentrations includes associating a desired temperature with a desired carbon dioxide concentration, with a desired carbon monoxide concentration, with a desired water concentration and with the ambient temperature.
 9. The method of claim 7, wherein controlling an airflow includes measuring the temperature of the output of the draft inducer.
 10. The method of claim 9, wherein measuring the temperature of the output of the draft inducer includes measuring with a thermistor.
 11. The method of claim 8, wherein associating a desired temperature with one or more concentrations and the ambient temperature includes accessing a look up table.
 12. The method of claim 1, further including determining the desired temperature to associate with the desired concentration.
 13. The method of claim 1, further including generating an audible signal.
 14. The method of claim 13, wherein the audible signal is generated by a speaker.
 15. The method of claim 13, wherein the audible signal is generated by a piezo alarm.
 16. A system for controlling the composition of the output of a draft inducer, the system comprising: means for associating a desired temperature of the draft inducer output with one or more desired concentrations of constituents of the draft inducer output; and means for controlling an air flow so as to substantially maintain the desired temperature.
 17. The system of claim 16, wherein means for associating includes a microprocessor.
 18. The system of claim 17, wherein the microprocessor contains a memory.
 19. The system of claim 18, wherein the memory includes a look up table.
 20. The system of claim 19, wherein the lookup table contains the desired temperature and the desired concentrations of the constituents.
 21. The system of claim 18, wherein the constituents include carbon dioxide, carbon monoxide, and water.
 22. The system of claim 16, wherein means for controlling an air flow includes a thermistor for measuring the temperature of the draft inducer output.
 23. The system of claim 16, further including means for measuring the temperature of ambient air entering the draft inducer.
 24. The system of claim 23, wherein means for measuring the temperature of ambient air includes a thermistor.
 25. The system of claim 23, wherein means for associating includes a microprocessor.
 26. The system of claim 25, wherein the microprocessor contains a memory.
 27. The system of claim 26, wherein the memory includes a look up table.
 28. The system of claim 27, wherein the lookup table contains the desired temperature, the desired concentrations of the constituents, and the ambient temperature.
 29. The system of claim 28, wherein the constituents include carbon dioxide, carbon monoxide, and water.
 30. The system of claim 16, further including means for generating an audible signal.
 31. The system of claim 30, wherein means for generating an audible signal includes a speaker.
 32. The system of claim 30, wherein means for generating an audible signal includes a piezo alarm.
 33. A draft inducer capable of coupling with an exhaust pipe, the draft inducer comprising: an input for detecting the temperature of a draft inducer output at a selected location in the exhaust port of the draft inducer; an air moving device for generating air flow in the exhaust pipe, the air moving device being capable of rotating at a plurality of different rotational speeds; and a control module for controlling the rotational speed of the air moving device as a function of the difference between the temperature of the draft inducer output at the exhaust port and a desired temperature reflective of desired concentrations of carbon dioxide and carbon monoxide at the exhaust port.
 34. The draft inducer of claim 33, wherein the control module controls the rotational speed of the air moving device to maintain the draft inducer output temperature at the exhaust port at substantially the desired temperature.
 35. The draft inducer of claim 33, wherein the draft inducer output is coupled to the exhaust pipe, the selected location being at a draft inducer exhaust port.
 36. The draft inducer of claim 33, further including an audible alarm.
 37. The draft inducer of claim 36, wherein the audible alarm is a piezo alarm.
 38. The draft inducer of claim 36, wherein the audible alarm is a speaker.
 39. A computer program product for use on a computer system for controlling the composition of the output of a draft inducer, the computer program product comprising a computer usable medium having computer readable program code thereon, the computer readable program code comprising: program code for associating a desired temperature with desired concentrations of one or more constituents of the draft inducer output; and program code for controlling an air flow so as to substantially maintain the desired temperature.
 40. The computer program product of claim 39, wherein the program code for associating a desired temperature with desired concentrations includes program code for associating a desired temperature with a desired carbon dioxide concentration, with a desired carbon monoxide concentration, and with a desired water concentration.
 41. The computer program product of claim 39, wherein the program code for controlling an air flow includes program code for measuring a temperature of the output of the draft inducer.
 42. The computer program product of claim 39, wherein the program code for controlling an air flow includes program code for accessing a look up table.
 43. The computer program product of claim 39, further including program code for measuring the temperature of ambient air entering the draft inducer.
 44. The computer program product of claim 43, wherein the program code for associating a desired temperature includes program code for associating a desired temperature with a desired carbon dioxide concentration, with a desired carbon monoxide concentration, with a desired water concentration and with the ambient temperature.
 45. The computer program product of claim 43, wherein the program code for controlling an air flow includes program code for measuring the temperature of the output of the draft inducer.
 46. The computer program product of claim 43, wherein the program code for associating a desired temperature includes program code for accessing a look up table.
 47. The computer program product of claim 39, further including program code for determining the desired temperature to associate with the desired concentration. 